Friend murine leukemia virus (F-MuLV) is a retrovirus obtained from stocks of Friend virus complex (F-MuLV plus spleen focus-forming virus). Each of these viruses can produce a fatal erythroleukemia upon injection into certain strains of mice. However, the erythroleukemia cell lines obtained from these animals have always been isolated from mice infected with both of these agents. We have developed a technique which enables us to reproducibly obtain erythroleukemia cell lines from mice infected with F-MuLV in the absence of any spleen focus-forming virus. These cell lines can be transplanted into syngeneic mice where they grown and produce a fatal leukemia. This technique involves the bi-weekly transfusion of RBCs into diseased mice infected with F-MuLV. These mice normally die from a profound anemia associated with the F-MuLV infection. However, the transfused mice live for longer periods. During this time they develop transplantable leukemic cells which we can isolate and grow in vitro. These tissue culture lines are erythroid in nature as evidenced by positive staining with an anti-spectrin antibody. They cause a donor cell leukemia in transplant recipients as shown by sex chromosome analysis. Future work will analyze the changes in the DNA of these leukemia cells which occur when the cells become malignant.